It depends on his mobility and focus. Going prone against a drive by is a fairly good idea, as it's tougher for him to stop and hard to aim at that speed. Going prone against an armed attacker on foot, not so much, if you go prone without cover he can just aim lower. Same thing goes for targeting - if you're caught in the crossfire, going prone is a good idea, you're less likely to catch a stray bullet and they have no reason to adjust their aim to hit a lower target. If you're the target, going prone is a crap idea, second only to standing around slackjawed. You'd be better off ducking and running. Of course, if you get into an actual situation like this, all bets are off.
I don't think anti-weapon skills would really add much. The main problems with sieges right now is that they're poorly executed. Goblins come in smaller numbers than you have (as a general rule, the sieging army should be two to three times the size of the defending army) and make a beeline for the nearest target. Meanwhile, the dwarves are feasting on ridiculously large harvests of mushrooms that require no fertilizer to produce. Historically, sieges were dangerous because they cut off supplies from the large fields around the castle. You can't threaten a society that can produce metric tons of food with little land and no sunlight with starvation, it just flat doesn't work. Making goblins better at defending against the WotM would just make the player diversify their army, and then the gobbos are still squishy.